Volatile fatty acid effects on nitrite removal and nitrate formation during activated sludge treatment

Reaction sequences of nitrifying and denitrifying bacteria are widely used to eliminate nitrogenous compounds from wastewater. Recent research showed that denitrification can occur under aerobic conditions. Volatile fatty acids (VFAs) were shown to reduce nitrate formation via nitrite oxidation in activated sludge systems and to stimulate aerobic denitrification in pure cultures. Nitrite removal inhibition by VFAs observed in activated sludge systems may be due to the level of aerobic denitrification. Investigation of this possibility can provide a new insight for the removal of nitrogen from wastewater and possibly reduce the chemical and energy demand for nitrogen treatment.; The impact of VFAs on nitrite removal and nitrate formation in activated sludge systems was studied in batch and CSTR experiments. The experimental work included measurements of nitrite removal, nitrate formation and CO 2 fixation in the absence and presence of VFAs. Additionally, molecular tools were applied to examine changes in microbial population density when the population was exposed to VFAs. Production of N2O and activity of periplasmic nitrate reductase enzyme (NAP) which catalyses the first step of aerobic denitrification were also analyzed.; Nitrite removal and nitrate formation rates were reduced in the presence of VFAs in batch experiments. Nitrate formation rate was reduced to a greater extent (74%) than nitrite removal rate (35%) indicating that products other than nitrate were formed during nitrite oxidation. The addition of VFAs into a CSTR resulted in a rapid decrease in nitrate formation rate (>70% reduction); however, nitrite removal rate was not reduced. No nitrogen was discharged in the effluent of the CSTR indicating that nitrogenous compounds were completely removed from the wastewater. In contrast, VFAs were not found to impact carbon dioxide fixation efficiency in either batch or CSTR experiments. Non-inhibitory effect of VFAs on carbon dioxide fixation implied that VFAs disturb nitrite removal and nitrate formation by a different system other than nitrite oxidation. N2O gas was produced in the presence of VFAs which was a clear indication that aerobic reduction of nitrite and/or nitrate occurred. It appeared that aerobic denitrification was responsible for the unbalanced nitrification conversions in the presence of VFAs.